1. Field of the Invention
The present invention relates to tractors for feeding continuous webs in printers and computers.
2. Description of the Prior Art
Such tractors may include a pair of side frames, a driving pulley rotatably supported between one ends of the side frames, a belt guide disposed between the side frames adjacent the driving pulley, and an endless feed belt trained around the driving pulley and the belt guide. The feed belt has on the outer surface thereof a predetermined number of feed pins engageable with equispaced perforations formed in a continuous web. The inner surface of the feed belt has a plurality of belt teeth spaced apart along the length thereof. The driving pulley has on the outer surface thereof pulley teeth adapted to mesh with the belt teeth on the arcuate driving portion of the feed belt. Thus, the feed belt may ba driven in a predetermined direction by rotation of the driving pulley, causing the continuous web to be fed in the predetermined direction.
The belt guide is usually formed integrally with one of the side frames, and has a guide surface having substantially the same width as the feed belt and extending in the travelling direction of the feed belt. The inner surface of the arcuate driven portion and the upper and lower reaches of the feed belt is guided in contacting engagement with the whole width of the guide surface of the belt guide.
In the foregoing web feed tractors, however, during driving of the feed belt, great friction is produced between the inner surface of the arcuate driven portion and the upper and lower reaches of the feed belt and the guide surface of the belt guide which is in contacting engagement with the whole width of the feed belt. Such a frictional force disadvantageously causes unstable travel of the feed belt and therefore, smooth web feed may not be effected.
Furthermore, in the prior art tractors, when the continuous web undergoes an external force greater than the meshing force between the belt teeth of the arcuate driving portion of the feed belt and the pulley teeth of the driving pulley, the arcuate driving portion of the feed belt is moved outwardly from the driving pulley, causing slip, jumping or other difficulties of the feed belt.